This invention relates in general to vacuum treating devices and in particular to a new and useful evaporator cell for applying thin layers on substrates by vacuum deposition.
By evaporator cell within the context of this specification, an evaporator in the form of a closed vessel is understood, from which the produced vapor escapes or is emitted through one or more apertures provided in the wall of the vessel.
Evaporative sources with movable screens are known thereabove, with which the escape way for the vapor stream can be opened or shut off. The vapor deposition time may thereby be adjusted to specific conditions, yet the rate of vapor emission from the source cannot be varied in this way.
Further known is an arrangement for controlling the rate of coating tapes by vapor deposition, providing adjustable screens between the evaporative source and the running tape, by means of which the tape portion to be exposed to the vapor stream is selected, then the time is defined during which each portion of the tape is coated. In this prior art arrangement again, not the vapor emission from the source but the deposition time for every portion of the tape is controlled.
To evaporate substances tending to sputter when fused, evaporators are sometimes employed in the art which are designed substantially as closed vessels having a wall portion with vapor escape openings (a screen). The wall prevents most of the spurting particles from reaching the substrates to be coated. To intercept even those which have passed through the screen into the deposition space, additional screens have been provided in front of the perforated wall portions, in staggered or offset position to prevent a straight-line passage of the substance to the substrates.
Also known are vapor sources or so called effusion cells, i.e. closed vessels in which the evaporative substance is heated, with the vapor escaping through an aperture provided in the wall of the vessel and having a diameter which is small relative to the mean free path of the vapor in the region of the outlet aperture.
Recently, more efficient evaporators have been developed for high evaporative rates, to improve the economy of coating. However, experience has shown that the high evaporative rates obtainable with such evaporators frequently cannot be utilized, because of their very high thermal intertia. For example, if layers of a mixture of different substances are deposited and the individual substances are evaporated in two or three separate evaporators, and if at the same time, the composition of the coating in the direction perpendicular to the substrate, is to be varied in accordance with a predetermined program, the problem arises that with too high evaporative rates, the mixing ratio cannot be varied quick enough. An adjustment of the evaporation, and the vapor deposition rate, which can be effected thermally by applying corresponding amounts of heat, leads to a time constant in the order of 10 to 60 seconds, which is too slow for economical manufacture. What is needed is rather to be able to adjust the evaporative rate to the specific coating requirements within substantially shorter time periods.